1. Field of the Invention
This application relates in general to drive systems for threaded fasteners, tools for their manufacture, and drivers for applying torque to such fasteners. More specifically this application relates to fasteners that employ winged type drivers. In particular fasteners having a recess that provides spiral driving surfaces.
2. Brief Description of Related Developments
Threaded fasteners commonly used in industrial applications typically are driven by power tools at high speeds and under high torque loads. Such conditions present difficult design considerations, particularly with respect to the drive systems and, more particularly, with threaded fasteners having a driver engageable recess in the fastener head. Ideally, such a drive system should be easily manufactured, both as to recess and head geometry as well as to associated tooling for forming the fastener head and drivers for engaging the recess. The strength of the head of the fastener should not be adversely affected by the recess. The driver should be easily insertable into and easily withdrawn from the recess. The driver and recess, when mated, should distribute the stress loads uniformly to avoid formation of highly localized regions of stress that might result in deformation of the recess, or driver, or both, leading to premature failure of the drive system.
The drive system should resist cam-out of the driver from the recess when the fastener is driven. In many applications, it is very important that the fastener must be capable of withstanding several cycles, as in applications where the fasteners must be removed in order to repair or replace parts or to remove and replace access panels. The fastener drive system ideally should be capable of such repeated cycling, particularly in environments where the recess may become contaminated, painted, corroded or otherwise adversely affected in use. In such environments, it is essential that the drive system maintain driving engagement while applying torque in a removal direction. It may be necessary for the drive system to be capable of applying even higher levels of torque when removing the fastener, as may occur when the fastener is over-torqued during initial assembly, or where corrosion develops at the interface at the engaged threads, or if thermal cycling of the assembled components has placed increased stress on the fastener. Where one or more of these, and other, characteristics may present competing considerations, compromises of one in favor of another may be made.
A variety of recess and driver configurations are in common use, including a number of cross-recesses, such as those described in U.S. Pat. No. RE-24,878 (Smith et al.); U.S. Pat. No. 3,237,506 (Muenchinger) and U.S. Pat. No. 2,474,994 (Tomalis). Other fastener geometries include multi-lobe geometries of the type described in U.S. Pat. No. 3,763,725 (Reiland) and ribbed drive systems as described in U.S. Pat. No. 4,187,892 (Simmons). Also among the common recess configurations is the Allen system which is essentially a straight walled hexagonally shaped socket receptive to a similarly shaped driver.
With the exception of the ribbed systems, the walls and faces of the driver and recess typically are designed to fit closely with each other in an effort to achieve face-to-face contact of the driving and driven surfaces. With cross-recess fasteners, such face-to-face engagement can occur only, if at all, when the driver is properly aligned and seated within the recess. As a practical matter, however, in order to enable the driver to be inserted into the recess, there necessarily must be some clearance between the two.
The necessity for such clearance is even more critical with recesses having substantially vertical drive walls, as in the Reiland ′725 patent and Allen head systems. In all of these systems, the practical result of the necessity for such clearance is that substantial face-to-face, broad area contact between the driver and recess surfaces is seldom achieved, if at all. With most drive systems for threaded fasteners, the driver mates with the recess in the head in a manner that results in point or line contact rather than face-to-face broad area contact. The actual area of contact typically is substantially less than full face-to-face contact. Consequently, when torque is applied by the driver, the forces applied to the screw head tend to be concentrated in localized areas with resulting high localized stresses. Such localized high stress can plastically deform the recess, forming ramps or other deformations resulting in premature, unintended disengagement of the driver from the recess. The foregoing difficulties have been recognized in the art. For example, U.S. Pat. No. 2,248,695 (Bradshaw) discloses a screw head and driver arrangement in which the driving and driven faces of the driver and fastener, respectively, are curved and located eccentrically with respect to the screw axis. In the Bradshaw fastener, any “suitable curvature” such as circular or log spiral may be used as long as it is oriented to bind or lock together by frictional engagement. Notwithstanding Bradshaw's teachings, later fastener drive systems, as those referred to above, do not appear to have adopted the Bradshaw teaching of relying on frictional engagement.
A drive system for maximizing the engageable surface area between the driver and recess is described in U.S. Pat. No. 5,957,645, which is commonly owned with the subject application. The disclosure of the ′645 patent is incorporated in this application by reference. The recess and driver of the ′645 patent are constructed with spirally configured engaging surfaces.
The advantages of the invention described in the ′645 patent are achieved by configuring the driving and driven surfaces of the driver and fastener, respectively, to conform to a segment of a spiral and, particularly, in a spiral configuration that enables a substantial and generous clearance between the driver and the recess during insertion and removal of the driver but in which the fully seated driver is permitted to rotate to take up that clearance. The spiral configurations of the drive walls of the driver and the driver-engageable walls of the recess are such that when the spiral walls engage, they do so over a relatively broad area thereby applying and distributing the stress over that broad area. The spirally configured driving and driven walls are oriented to direct a major portion of the applied torque substantially normal to the fastener radius with little, if any, reliance on frictional, near-tangential engagement.
In many applications in which a spirally configured drive system is used, the driver may be power driven or required to be inserted in locations of limited access.  In such situations, there is a need to releasably engage the fastener on the driver in advance of installation so that the driver can be used as an insertion tool, as well as a driver. This “stick fit” feature has been attempted in several different types of fasteners, for example, as shown in U.S. Pat. Nos. 6,199,455; 4,457,654; and 4,084,478.
In the ′478 patent a series of ribs 26 are inserted in the recess to provide the interference fit. In the ′654 patent a cap is installed over the screw head, pressed into the recess and a detent, annular ridge 30, is provided in the cap to engage the driver. In the modified square drive fastener of the ′455 patent, adjoining outer walls 17 and 23 are tapered to provide a stick fit feature. The surfaces 17 and 23 are selected for their larger area relative to driving surfaces 14.
The “stick fit” feature allows the fastener to be releasably engaged on the driver to enable manipulation of the driver and fastener as a unit in hard to reach, automated, and other applications. Once installed, the fastener and driver may be disengaged with minimal effort.
The ′645 patent indicates that the a positive draft may be formed in the installation, removal, and other surfaces of the recess. Although it isn't clear from the description, such a draft may be for the purpose of facilitating the engagement of the driver in the recess or providing a degree of latitude for the driver orientation other than straight axial alignment. There is no indication that such a draft would be used for the purpose of providing a “stick fit” feature on this type of fastener. In fact the suggested draft of up to 6 degrees, is far in excess of what would be needed and would significantly interfere with the purpose of the spiral installation surface, which is namely to provide engagement of the driver and fastener along an extended surface area as opposed to the tangential engagement of other drive systems. It is therefore needed to provide a suitable means of providing a stick fit feature on a spirally configured fastener.